Igneous extrusive stone layers are formed from andesite, basalt, dacite, obsidian, and rhyolite. Of these five, andesite, dacite, and rhyolite are functionally identical, different only in appearance and name, basalt is like the other three with the caveat that it is magma-safe, and obsidian is... unique.

In real-world geology, Igneous extrusive stone is formed by lava solidifying above the surface, open to the air. This is the main point of difference from igneous intrusive stone, which solidifies below the surface. This distinction is not kept in Dwarf Fortress geology, in which the difference is simply the name and contents of the two types of stone.